Papermaking involves the forming, pressing and drying of cellulosic fiber sheets. The forming process includes the step of depositing an aqueous stock solution of the fibers, and possibly other additives, onto the forming fabric upon which the initial paper web is formed. The forming fabric may run on a so-called Gap Former machine in which the aqueous stock initially is dewatered, and the initial paper sheet is formed between two forming fabrics.
An effective forming process typically produces a sheet with a very regular distribution of fibers and with a relatively high solids content, i.e., a high fiber-to-water weight ratio. In order to form a fibrous web with a desired uniform, regular distribution and high fiber-to-water weight ratio, the forming fabric must possess a number of properties. First, the papermaking surface should be relatively planar; resulting from the yarn floats in both the machine direction (MD) and cross-machine-direction (CD) lying at substantially the same height, to thereby prevent localized penetration of the fibers into the fabric. Such localized penetration results in “wire marks” which actually are the result of fiber density variations throughout the sheet area. In addition, the MD and CD floats need to be distributed in a regular manner to avoid introducing undesired wire marks into the formed sheet. Moreover, these basis weight variations can result in undesired variations in sheet absorption properties; a property very relevant to the functionality of quality graphical papers where a consistent uptake of print ink is necessary to produce a clear sharp image.
Other factors also cause the formation of undesired wire marks. For example, wire marks can be introduced into the sheet by the flow of water around yarns positioned below the fabric's papermaking surface. This phenomena, referred to as “strike through,” needs to be taken into account in designing the fabric construction.
Importantly, the forming fabric must also possess a high degree of dimensional stability. This high stability is necessary, for example, to minimize cyclic variations in fabric width, which can result in MD wrinkles in the formed paper web. This, in turn, contributes to the so-called, streaky sheet, i.e., a sheet with machine direction streaks created by variations in fiber density.
Dimensional stability of a fabric typically is obtained by manufacturing the forming fabric with a relatively high mass of material. However, the use of thick yarns to establish high mass often causes undesirable wire marks. Consequently, there has been a trend to providing composite forming fabrics, that is, “multi-layer” structures, whereby a high number of relatively thin yarns are distributed throughout various fabric layers to facilitate fabric stability.
One type of multi-layer fabric is a triple-weft fabric made by interlacing one machine direction yarn system with three (3) cross-direction yarn systems. Such a fabric structure is taught in U.S. Pat. No. 4,379,735, issued to McBean. The three cross-direction yarn systems are arranged so that one system interlaces with the machine direction yarn system to form the paper side of the fabric; one system interlaces with the machine direction yarn system to form the wear side of the fabric, i.e., the side in contact with the paper machine dewatering elements, e.g., vacuum boxes, and the third cross-direction yarn system interlaces with the machine direction yarn system while at all times being positioned vertically between the paper side cross-direction yarn system above and the wear side cross-direction yarn system below. Accordingly, in all the triple-weft fabrics the same machine direction yarns interlace with both the paper side and wear side cross-direction yarn systems. This results in the machine direction yarns forming part of the fabric's paper side and wear side surfaces. This triple-weft fabric system requires a significant compromise in choice of MD and CD yarn diameters to attempt to meet the different requirements of the paper side and wear side surfaces of the forming fabric. However, the triple-weft structure does provide a very high CD bending stiffness and with it the ability to reduce sheet basis weight profiles.
Another type of multi-layer structure is a triple-layer fabric made by joining two (2) distinct fabrics, each with their own machine direction (warp) yarns and cross-direction (weft) yarns, by the use of additional and independent “binding yarns.” These binding yarns can be employed in either the machine direction or cross-machine-direction, and in this system provide the sole function of binding the two separate fabrics together. In other words, these binding yarns are not intended to function as part of the warp or weft yarn system in either the top fabric or the bottom fabric of the multi-layer structure. Such a triple-layer fabric is illustrated in EP 0,269,070(JWI Ltd.).
Where the two fabrics of the triple-layer structure are joined in either the machine direction or cross-machine-direction by binding yarns that also belong, or form part of the weave pattern of either, or both, the paper side or wear side fabrics, the resulting structures are referred to more specifically as “self-stitched” triple-layer structures. Such binding yarns are referred to as “intrinsic binding yarns.” Self-stitched structures are taught in a number of prior art patents. For example, U.S. Pat. No. 4,501,303 (Nordiskafilt AB) discloses a triple-layer structure wherein paper side yarns are used to bind the paper side and wear side fabrics into one structure.
Triple-layer structures, whether employing separate and distinct binding yarns or intrinsic binding yarns that form part of the paper side and/or wear side weave structure, allow, to some extent, for the use of fine machine direction and cross-machine-direction yarns in the paper side fabric for improved papermaking quality and sheet release. In addition, significantly coarser yarns can be employed in the lower fabric, or wear side fabric, which contacts the paper machine elements, to thereby provide good stability and fabric life. Thus, these triple-layer structures have the capability of providing optimum papermaking properties in the paper side fabric and optimum strength properties in the wear side layer. However, in comparison with the aforementioned triple-weft structures, in the triple-layer structures the CD bending stiffness is reduced; thereby reducing the ability to minimize sheet basis weight profiles.
A variety of composite fabrics employing intrinsic interchanging yarn pairs have been disclosed to attempt to deal with the various problems of fabric strength, fabric stability e.g., fabric stiffness, desired paper side performance and desired wear side performance. In particular, various different composite fabric constructions are disclosed in U.S. Pat. No. 4,501,303 (Osterberg); U.S. Pat. No. 5,152,326 (Vochringer); U.S. Pat. No. 5,826,627 (Seabrook et al.); U.S. Pat. No. 5,967,195 (Ward); U.S. Pat. No. 6,145,550 (Ward) and International Publication WO 02/14601 A1 (Andreas Kufferath GMBH&Co. KG). In all of these structures, all of the interchanging yarn pairs cooperate to provide an uninterrupted or continuous weave pattern in each repeat of the fabric weave pattern of the paper side layer; preferably a continuous plain weave structure. The continuous, uninterrupted plain weave pattern in prior art structures is established by one yarn of the pair moving out of the paper side layer on one side of a single, paper side warp transition yarn and the other yarn of the pair moving into the paper side layer on the opposite side of the single warp transition yarn.
If there were two or more contiguous paper side warp transition yarns between the location where one yarn of the pair moves out of the paper side layer and the other yarn of the pair moves into the paper side layer the plain weave pattern provided in each segment of the paper side layer by each respective yarn of the pair would be interrupted, or rendered discontinuous at the interchange location. Likewise, if one yarn of the pair overlies the other yarn of the pair in the paper side layer without the provision of a paper side warp transition yarn, then the plain weave pattern also is interrupted, or rendered discontinuous.
U.S. Pat. No. 5,437, 315, issued to Ward, discloses a triple-layer fabric having both a top fabric layer and a bottom fabric layer, each including machine direction yarns interwoven with cross-machine-direction yarns. Weft binder yarns, which have a number of top weft yarns between each successive pair, are spaced-apart from each other in the machine direction, extend generally parallel with the cross-machine-direction yarns of the top fabric layer and the bottom fabric layer and interweave with the top fabric layer and bottom fabric layer. In the disclosed structures, each of the spaced-apart binder yarns replaces a cross-machine-direction yarn of the top fabric layer when the binder yarn engages one or more machine direction yarns of the top fabric layer.
The requirement in prior art structures that the interchanging yarn pairs provide a continuous weave pattern imposes limitations on establishing the desired fabric stiffness of the fabric whilst maintaining sufficient openness of the fabric's paper side surface to allow for the passage of the required amount of water at optimal machine running speeds.
Although the aforementioned composite papermaking fabrics employing intrinsic interchanging yarn pairs have provided improved structures, applicants believe that there still is a need for additional, improved composite structures of the type employing intrinsic interchanging yarn pairs having a desired balance of sheet dewatering properties, high resistant to layer delamination and stability for sheet basis weight control. It is to such structures that the present invention is directed.